Observed secretly (5)
I believe the answer is:
spied
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">observed secretly</span>' is the definition.<br/ ><span class="explanation-format__explanation">('spy' can be a synonym of 'observe secretly')</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for spied that I've seen before include "Watched secretly" , "Caught sight of" , "Was a secret agent" , "Saw" , "Seen" .)